This invention relates to projectiles and to a protective nose cap structure for projectiles which are handled under conditions involving a risk of being accidentally dropped or otherwise impacted.
Rough handling of artillery projectiles is commonplace due to their high weight and bulk, plus the fact that orderly and deliberate handling procedures are not characteristics of battlefield conditions. When projectiles are dropped on a hard surface such as concrete, concentrated local stress reaching a high level is produced at the point of impact. While the projectile walls are necessarily strong and may be expected to resist deformation, actual or incipient cracking of the wall is occasionally experienced.
Discontinuities, cracks or crack-like defects, regarded as possible failure initiation sites inherent in the projectile material, may or may not be detectable upon inspection. Limits for acceptable flaw sizes in projectiles are defined and accordingly projectiles are screened for field use. The problem exists however, that after a projectile passes inspection and in the course of routine handling prior to firing an innocuous flaw size can become a critical flaw size, and thus become a "primed" failure initiation site capable of producing premature projectile failure under the normal load conditions of firing. Such growth, or enlargement of flaws can occur under conditions where a high, localized and crack-opening stress is encountered, as in the case of impact. As materials, generally, possess a crack size tolerance, or "fracture toughness", for a given load or stress level, the projectile material has a tolerable crack size than can safely survive the launch stresses. The more brittle, or the more of a fragmentation quality, a projectile is, the smaller is the crack size that it can tolerate at a given stress--the "primed", or critically-sized flaw may be such, that not only will it readily escape visual detection, but also, that it can escape detection by more precise inspection techniques. In any event, the firing of a projectile with a "primed" or critically-sized flaw will result in a mechanically-unstable projectile incapable of withstanding the stresses encountered in firing; this could result in a premature, or an in-bore failure of grave consequence.
Experimental stress analysis of projectiles, having a lift plug screwed into the fuze well, subjected to simulated rough handling in accordance with the U.S. Army Test and Evaluation Command Material Test Procedure-MTP 4-2-602, "Rough Handling Tests", quantitatively showed the fuze well of the projectile to be especially vulnerable to this sort of structural instability. The invention described herein, presents a solution to this problem.